There are two main catches to forclosures. One is obvious, if someone knows they are going to be forclosed on, they often trash the place, sometimes for spite, but usually they take all the appliances, light fixtures etc. so they can salvage something of what they've lost. You are buying "as is," so you will usually have to buy all new appliances and may have to make more major repairs.
The other problem a lot of people have with forclosures is that the banks will not accept a contingent contract. That means if you are selling a house and want to use the money you make from it as a downpayment on a new one, you already have to have the money in hand to buy a forclosed property.
A couple more issues pop up with tax sale forclosures. One is that the people are often still in the house, so you can't go see it. Another is they have a time period after the sale (30 days??) where the existing property owner can still pay the taxes and keep their property. You have to put down a deposit at the time of the sale but you will get it back if the sale gets canceled.
Those are the "catches" I know about, but if you have the down payment and are able to preapproved for a loan. You are not worried about selling another house but have some place to stay until the sale goes through, you can get some really great deals.